A sheathed heater in which nichrome wire is employed has been widely used as a heat source in electric cookers, electric water heaters and the like. This sheathed heater performs heating by inserting nichrome wire into a metallic sheath tube, filling magnesia powder or the like into a space in the tube, sealing tightly, and supplying electric current through the nichrome wire. This heating method is very safe since no flame is used, and has been widely employed in electric cookers such as fish baking grills, electric water heaters and the like as a necessary item for a so-called all-electric home. The demand for this has become very widespread (See Japanese Examined Patent Application Publication No. Sho64 (1989)-008695, No. Sho64 (1989)-011106, Japanese Unexamined Patent Application Publication No. Sho63 (1988)-121641, No. 2013-241650, and No. 2014-84493).
However, Fe—Cr—Ni—Mo alloy containing Ti and Al which is a necessary component for a sheathed heater has a problem in that surface defects may occur since Ti and Al would cause generation of TiN or alumina inclusions. To solve this problem, a technique is disclosed in which Si concentration is decreased so as to control generation of TiN. However, there is another risk of the occurrence of defects by non-metal inclusions of oxide composition (See Japanese Unexamined Patent Application Publication No. 2003-147492).
Furthermore, a technique to produce Fe—Cr—Ni alloy having superior surface property is disclosed. This technique reduces MgO.Al2O3 (spinel type) and CaO inclusions so as to prevent surface defects. This technique controls the inclusions as CaO—TiO2—Al2O3 type inclusions; however, inclusions mainly containing TiO2 may be generated depending on condition of operation, and there may be defects generated. In particular, since the sheathed heater material requires strict surface quality, the technique cannot be employed (See Japanese Unexamined Patent Application Publication No. 2014-189826).